Semiconductor chips are commonly provided in packages that facilitate handling of the chip during manufacture and during mounting of the chip on an external substrate such as a circuit board or other circuit panel. For example, many semiconductor chips are provided in packages suitable for surface mounting. Numerous packages of this general type have been proposed for various applications. Certain types of packages have been developed which utilize a microelectronic component having a flexible dielectric substrate having conductive traces disposed thereon. In such an arrangement, electrically conductive posts or pillars project from a surface of the flexible substrate. Each post is connected to a portion of one of the traces. This type of microelectronic component is particularly useful in chip packages having arrangements that allow each post to move independently of the other posts. The movement of the posts allows the tips of the plural posts to simultaneously engage contact pads on a circuit board despite irregularities in the circuit board or the package, such as warpage of the circuit board. Additionally, this facilitates testing of the package using simple test boards which may have substantially planar contacts, and avoids the need for specialized, expensive test sockets.
This type of microelectronic component has various applications and can be used in a number of different microelectronic package arrangements. As disclosed in certain preferred embodiments of U.S. patent application Ser. Nos. 11/014,439; 10/985,119; and 10/985,126, the disclosures of which are incorporated by reference herein, one such microelectronic package can include a microelectronic element such as a semiconductor chip and a microelectronic component comprising a flexible substrate spaced from and overlying a first face of the microelectronic element. Such a component can include a plurality of conductive posts extending from the flexible substrate and projecting away from the microelectronic element, at least some of the conductive posts being electrically interconnected with said microelectronic element. Such posts are typically fabricated from a solid metal, which is typically either copper, copper alloy, gold or combination of these materials. Additionally, such a package can include a plurality of support elements disposed between the microelectronic element and the substrate and supporting the flexible substrate over the microelectronic element. Desirably, at least some of the conductive posts are offset in horizontal directions parallel to the plane of the flexible substrate from the support elements. For example, the support elements may be disposed in an array with zones of the flexible substrate disposed between adjacent support elements, and the posts may be disposed near the centers of such zones.
The offset between the posts and the support elements allows the posts, and particularly the bases of the posts adjacent the substrate, to move relative to the microelectronic element. This arrangement can allow each post to move independently of the other posts. This movement of the posts is provided by the low modulus of elasticity of the flexible substrate. Typically, the flexible substrate is made from polyimide, which has a modulus of elasticity of about 2 GPa to 5 GPa. The modulus of elasticity of the conductive posts, by comparison, is typically about 120 GPa, which means that in such a package the conductive posts are effectively rigid and unyielding. Further, such an arrangement effectively has no range of plastic deformation because the stress required to achieve plastic deformation of a solid metal pin exceeds values that would likely damage the semiconductor component of a printed circuit board (PCB) to which the structure is mated.
The flexible substrate can overlie the front or contact-bearing face of the microelectronic element. In this arrangement at least some of the support elements desirably are electrically conductive elements such as solder balls. The conductive support elements may electrically interconnect at least some of the contacts of the microelectronic element with at least some of the conductive posts. In preferred forms, this arrangement can provide low-impedance conductive paths between the posts and the microelectronic element, suitable for high-frequency signal transmission. At least some of the posts can be connected to at least some of the contacts on the microelectronic element by conductive support elements immediately adjacent to those posts. It is advantageous that conductive traces provided on the flexible substrate electrically interconnect at least some of the conductive posts with at least some of the conductive support elements. These traces may be very short; the length of each trace desirably being equal to the offset distance between a single post and a single support element.
The flexible dielectric substrate utilized in such a microelectronic component includes a top surface and a bottom surface remote therefrom. Although the thickness of the dielectric substrate will vary with the application, the dielectric substrate most typically is about 10 μm-100 μm thick. The flexible sheet has conductive traces thereon. In one embodiment the conductive traces are disposed on the bottom surface of the flexible sheet. However, in other embodiments the conductive traces may extend on the top surface of the flexible sheet, on both the top and bottom faces or within the interior of flexible substrate. The thickness of the traces will also vary with the application, but typically is about 5 μm-25 μm. Traces are arranged so that each trace has a support end and a post end remote from the support end.
The dielectric sheet, traces and posts can be fabricated by a process such as that disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 10/959,465, the disclosure of which is incorporated by reference herein. As disclosed in greater detail in the '465 Application, a metallic plate is etched or otherwise treated to form numerous metallic posts projecting from the plate. A dielectric layer is applied to this plate so that the posts project through the dielectric layer. An inner side of the dielectric layer faces toward the metallic plate, whereas the outer side of the dielectric layer faces towards the tips of the posts. Previously, this dielectric layer has been fabricated by forcibly engaging the posts with the dielectric sheet so that the posts penetrate through the sheet. Alternatively, the dielectric sheet may be provided with an array of pre-formed holes at the locations of the posts, and attached to the plate with an adhesive. Once the sheet is in place, the metallic plate is etched to form individual traces on the inner side of the dielectric layer. Alternatively, conventional processes such as plating or etching may form the traces, whereas the posts may be formed using the methods disclosed in commonly assigned U.S. Pat. No. 6,177,636, the disclosure of which is hereby incorporated by reference herein. In yet another alternative, the posts may be fabricated as individual elements and assembled to the flexible sheet in any suitable manner that connects the posts to the traces. The composition and design of these posts, in combination with the methods used to either form them with the microelectronic component or to assemble them therewith, require narrow tolerances in order to ensure that the respective heights of the posts above the dielectric layer are substantially equal. Such narrow tolerances lead to increases in both the cost of the microelectronic packages and the time required for their manufacture.
Despite the aforementioned advances in the art, still further improvements in microelectronic components would be desirable.